Urinary tract infections (UTIs) represent the second most common infectious disease in the United States and a significant cause of morbidity, with major economic and social impact. These infections commonly are caused by the Gram-negative bacterium Escherichia coli. Once uropathogenic E. coli (UPEC) is introduced, survival and persistence of these bacteria in the urinary tract require a specific set of virulence factors. Recent work in the murine cystitis model has revealed a pathogenic cascade of events in E. coli UTI, including bacterial attachment and internalization, followed by the formation of intracellular biofilm-like pods of bacteria within superficial facet cells of the uroepithelium. Host responses include uroepithelial production of inflammatory cytokines and an influx of neutrophils into the bladder. UPEC strains attenuate epithelial cytokine production sharply in vitro, suggesting a mechanism by which UPEC may attempt to subvert this immune response to gain a foothold within the bladder. In addition, UPEC multiply to high levels within the mouse bladder despite the presence of a significant neutrophil infiltrate. We hypothesize that UPEC employs strategies to attenuate host cytokine signaling, resist phagocytosis, survive within phagocytes, and/or affect the generation of reactive oxygen species within neutrophils. We further hypothesize that arriving neutrophils respond to specific chemical signals to target only pod-infected uroepithelial cells. This proposal blends in vitro neutrophil and tissue culture assays, bacterial genetics, the murine cystitis model, quantitative gene expression profiling, and proteomics to identify the molecular determinants of the interactions between UPEC and components of host immunity, particularly neutrophils. The results will advance our understanding of host-pathogen interactions within the bladder, offer targets for new therapeutic and preventive strategies for UTI, and provide a foundation for new paradigms in host-pathogen crosstalk that can be generalized to other Gram-negative bacterial infections.